Ledmasters has created a chronograph for measuring bullet speed, using a Teensy 4.0 connected to an Adafruit Feather M0 via RFM69 433MHz radio modules.

Stats are displayed on a 3.2″ TFT display, and can be printed via an integrated thermal printer. Piezos attached to PET film are used to measure the bullet strike and elapsed time over a 12″ span. More information about the project can be found in the original forum thread.